Refrigerated display cabinets which have a front opening and multiple air curtains to isolate the refrigerated space from ambient atmosphere are well known. With this type of cabinet, refrigerated foods and other merchandise can be easily removed from or placed in the refrigerated space.
Such cabinets have gained wide acceptance in the food inustry. Air provides an innermost curtain and at least one adjacent outer air curtain across the front opening. The air curtains are normally circulated through conduits provided within the display cabinet. The innermost air curtain is normally the coldest and the other air curtains are somewhat warmer. A refrigerating means, including one or more evaporators, is located in the innermost curtain conduit for cooling the air.
In this type of refrigerated display cabinet, the innermost curtain conduit and refrigerating means must be defrosted periodically to remove accumulated frost from the evaporators. This frost, formed from the circulating air, impedes the operation of the equipment. Commercially, defrosting has been achieved by high voltage electrical heaters located adjacent the evaporator. With electrical heater defrosting, the refrigerating operation is temporarily halted while air continues to circulate. Thus, the circulating air is warmed by the electrical heater. This warm air melts frost built up on the evaporator. It is important to melt this frost as rapidly as possible in order to minimize temperature rise of the refrigerated goods and to minimize frost collection on the refrigerated goods caused by the higher humidity of the recirculated warm air. In some instances, a hot gas is passed through the evaporator of the refrigerating means. However, the hot gas defrost method is complex in its construction and is practical in only a small percentage of installations.
One method of resolving the above disadvantages is disclosed in Maehara et al., U.S. Pat. No. 4,644,758 issued on Feb. 24, 1987. As shown in FIG. 1, the refrigerating apparatus of this prior invention includes a compressor 1, a condenser 2, and two evaporators 3a and 3b each of which are serially connected to compressor 1 and condenser 2. The suction sides of both evaporators 3a, 3b are connected to condenser 2 through valve devices 4a, 4b, respectively, and are connected to each other by a first passage line which includes first and second expansion valves 5a, 5b and first and second check valves 6a, 6b, respectively, in series. The discharge sides of both evaporators 3a, 3b are also connected to each other by a second passage line which includes a pair of check valves 6c, 6d. This second passage line is connected to the compressor through valve devices 4c and 4d. The first and second passage lines are connected to each other and to condenser 2 through a valve device 4e.
In this construction of a refrigerating apparatus for a refrigerated display cabinet, if valve devices 4c, 4d, 4e are opened, the refrigerant passes through both evaporators 3a, 3b, which are connected in parallel, while being expanded within the evaporators as indicated by the solid arrows in FIG. 1. Therefore, the air passed through the evaporators is cooled by indirect heat exchange with the refrigerant.
If valve devices 4a and 4d only are opened, evaporators 3a and 3b are serially connected to each other, and condensed refrigerant is passed first through evaporator 3a and then through expansion valve 5b and evaporator 3b as indicated by the dotted and dashed arrows in FIG. 1. Therefore, evaporator 3a is defrosted by the condensed (unexpanded) refrigerant, and air passing through evaporator 3b is cooled by the expanded refrigerant. Conversely, if valve devices 4b and 4c are opened, evaporator 3b is defrosted. Therefore, one evaporator can be partially defrosted by the condensed refrigerant while air is refrigerated in the other evaporator by the expanded refrigerant.
However, the defrost operation in this refrigerating apparatus is insufficient. In the defrost mode, heated and highly pressurized condensed refrigerant is introduced into the evaporator at its upper terminal end allowing the upper portion of the evaporator to be defrosted by condensed refrigerant. However, refrigerant with a lower temperature is passed through the lower portion of the defrosting evaporator. This lengthens the time required to completely defrost the lower portion of the evaporator.
Also, after the frost is melted, the surface temperture of the evaporator is raised to the temperature of the condensed refrigerant. As a result, the air temperature on the peripheral portion of the evaporator is raised. But this phenomenon only occurs on the upper portion of the evaporator. Heated air does not effectively melt the frost on the lower portion of the evaporator. Furthermore, heated air remains in the space between the upper surface of the evaporator and a shutter element. Therefore, if the sealing between the shutter element and the air passageway or heat insulation of the shutter element is insufficient, the heated air is transmitted to the cold air passed through the cooling passageway.
Moreover, in this refrigerating apparatus, each evaporator has a corresponding expansion valve. The expansion valves are operated under different conditions: the full refrigerating mode or the defrosting mode. The refrigerant flow for these conditions is different. If the capacity of the expansion valve is sufficient for the refrigerating mode, the capacity is insufficient in the defrosting mode. Conversely, if the capacity of the expansion valve is sufficient for the defrosting mode, the capacity is in excess of that required in the refrigerating mode. This type of operation of the expansion valve is unstable and causes a large load to act on the compressor. Also, this operation increases manufacturing costs of the refrigerating apparatus.